1. Field of the Invention
The present invention relates to an optical system that has a plurality of lenses of two different focal lengths in a digital camera (hereinafter referred to as DSC), and by using a parallelogram prism (rhombic prism) or a pair of triangular prisms those two lenses can be placed in the image capturing position interchangeably.
2. Background Art
To obtain different focal lengths in a camera, usually zoom lenses, multifocal lens turrets and conversion lenses have been commonly used. The zoom lenses are relatively expensive and constructions are complicated and difficult to be made. The lens turret system is much easier than zoom lenses in manufacturing, however, longer focal lenses are always protruded and cannot be retracted in to the camera body, and that makes the camera bulky. There are two kinds of conversion lens. One is to be attached to the front of camera lens, of which the focal magnification is very limited due to its size, and it is extremely difficult to obtain a focal magnification of two times. This type of front conversion lenses is too bulky for a DSC, because usually the sizes of DSC lenses are much smaller than analogue camera lenses, and when used in a DSC, the durability of the total lens with a front conversion lens will be in question. The other is a behind lens type conversion (a rear conversion lens), which is to be inserted behind the master lens to change the focal length of the master lens, but also the ratio of focal magnification is limited, and further more the mechanism to insert the rear conversion lens behind the very small sized DSC master lens is very complicated and not easy to be designed and manufactured.
FIG. 1 shows a typical design of lens turret system, of which telephoto lens is protruded from the camera, and FIG. 2 shows an example of a front conversion lens to be attached to a camera, which inevitably becomes quite large.
It is therefore an object of this invention to provide a multifocal lens system which is much less expensive and easier in manufacturing than zoom lenses, and not so bulky as lens turret systems and front conversion lenses, and not so complicated in mechanism as rear conversion lenses, and yet which can easily obtain large magnification like 3 times or over.